<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <style>
            body {
                font-size: 60px;
            }

            .box1 {
                width: 200px;
                height: 200px;
                background-color: #bfa;
                position: absolute;
                /* 
                  对于开启了定位元素,可以通过z-index属性来设置元素的层级
                      z-index需要一个整数作为参数,值越大的元素层级越高
                        元素的层级越高越优先显示

                      如果元素的层级一样,则优先显示靠下的元素
                      
                      祖先元素的层级再高,也不会盖住后代的元素
                */
                /* z-index: 1; */
            }

            .box2 {
                width: 200px;
                height: 200px;
                background-color: rgba(255, 0, 0, 0.5);
                position: absolute;
                top: 50px;
                left: 50px;
                /* z-index: 2; */
            }

            .box3 {
                width: 200px;
                height: 200px;
                background-color: yellow;
                position: absolute;
                top: 100px;
                left: 100px;
                z-index: 3;
            }

            .box4 {
              width: 100px;
              height: 100px;
              background-color: #eea216;
            }
        </style>
        <title>Document</title>
    </head>
    <body>
        <div class="box1">1</div>
        <div class="box2">2</div>
        <div class="box3">
            3
            <div class="box4">4</div>
        </div>
    </body>
</html>
